A disk drive is made up of a platter, an arm with a read/write head at the end of it, some gizmo to move the arm back and forth, and some electronics to control it all. Platters are made up of a magnetic material. You can think of it as being made up of billions of little magnets, some with the north pole up (for zeros), some with the south pole up (for ones). The disk drive head floats just above the platter. To read from the disk, a coil in the head can sense the orientation of each tiny magnet, and convert it into a one or zero. To write to the disk, the orientation of the individual little magnets is set by passing an electrical current through the coil. How a Hard Drive Works* *it's actually more complicated, but this is the basic idea...

Tracks and Sectors a Track is a ring on the surface of the disk, like the ring of a tree a Sector is a pie-shaped wedge of a track. Sectors are generally 512 bytes. Data on disk drives is always read and written in blocks of 512. Seek time is the amount of time it takes for the read/write head to move to the right sector on the disk to find the data you want. It depends on how long it takes the head to move back and forth, and how fast the disk rotates.

Deleting Files The operating system keeps a list of the free sectors that are available on the hard drive. When you save information to a file, it picks some sectors from this list and writes your information into them. It stores information about the name of the file and the sectors that file uses in a directory, which is also kept on the disk. When you move a file to the recycle bin, the entry in the directory is just marked in some special way – you can recover it if you change your mind. When you delete the file from the recycle bin, the sectors that it used are put back on the free list, to be reused for some other file. The information in the sectors is not actually erased!

Defrag ment ing If a file is composed of sectors that are spread out all over the disk, it can take a while for the operating system to read it, because the read/write head has to jump around a lot. When this happens, we say that the file is fragmented. After the disk has been used for a while and starts to get full, this can happen frequently, as new files are created from the sectors released from other deleted files. The operating system gives you a way to defragment the drive – to reorganize the files so that the sectors for each file are contiguous – right next to each other on the platter.